| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 90 === The numerical simulation adopted in the present study deals with the investigation of unsteady flowfield characteristics that induced by interactions between the centrifugal impeller and the volute. Effects of the flowfield on impeller performance, vibration, and noise production have therefore been examined. The finite volume method is employed to analyze the three-dimensional Reynolds-averaged Navier-Stokes equations. In a centrifugal turbomachine, the steady calculation of a single impeller passage, as well as the quasi-steady and unsteady calculations of the impeller-volute flowfield are performed respectively. The obtained numerical results are then compared with an experimental measurement to analyze the flowfield phenomena inside the impeller. In addition, effects of the presence of volute on the overall performance, impeller radial loading, and the mechanism of noise production are all examined in detail. The results indicate that the computation considering both the impeller and volute better approximates to realistic physics, and therefore leads to a more accurate assessment of overall performance. Furthermore, some other important findings can be obtained from the observation of the volute flowfield. For the flow rate at the off-design point, the divergent angle of volute is found unable to match that of the flow rate. This thus results in a non-uniform volute pressure distribution, and a subsequent unbalanced impeller radial loading. Additionally, the observation of the flowfield in the vicinity of the volute tongue exhibits a marked pressure fluctuation, which is observed near the tongue. This is caused by the interactions of the blade and volute, and is mainly responsible for noise production. Furthermore, calculations for both various impeller radiuses, and the tongue clearance are conducted. The impending result shows that overall performance is actually degraded with a reduced impeller radius and an increased tongue clearance. As the impeller size is reduced, the design point flow rate, which original gives the minimum impeller loading, results in a larger impeller loading. However, the lower flow rate case originally giving the larger impeller loading alternatively leads to the gradually reduced loading under this situation. Meanwhile, according to the wall pressure variation amplitude (near the tongue), increasing the tongue clearance allows for both pressure fluctuation decreases, as well as further aiding in noise reduction.
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